Three dimensional characteristics of mountain waves generated by the Sierra Nevada

The Sierra Nevada mountain range is a north-northwest to south-southeast oriented mountain range of approximately 650 km length, 100 km width, and features the tallest peak, Mt. Whitney (4,417 m), and the steepest orographic gradient along the eastern slope in the contiguous United States. The Sierra Nevada is well known for generating large amplitude mountain waves (i.e., the Sierra Wave). Although the Sierra Nevada is often considered nearly a two-dimensional barrier to southwesterly flow, the range has a number of peaks above 4 km along the crest and several deep passes.

Measurements from the NSF/NCAR High-performance Instrumented Airborne Platform for Environmental Research (HIAPER) obtained during the recent Terrain-Induced Rotor Experiment (T-REX) indicate marked differences in the character of the wave response between northern and southern cross Sierra tracks, which were separated by a distance of approximately 50 km. Observations from several of the HIAPER research flights indicate that the vertical velocities in the primary wave exhibited variations up to a factor of two between the southern and northern portion of the racetracks in the lower stratosphere. The perturbation wind speed and potential temperature also exhibited large differences in the direction along the Sierra crest. Multiple racetracks at the 37000 ft. and 43000 ft. altitudes indicate that these differences were repeatable, which is suggestive that the deviations were likely due to mountain waves that varied systematically in amplitude rather than associated with transients. Some of the research flights indicated that the largest variations were on the northern flight leg, which traversed above Independence, CA, while other flights showed larger variations on the leg to the south. The topography beneath the northern flight segments is typically higher than the southern flight segments. However, a number of topographic peaks near the southern segment are among the highest in the Sierra.

In this study, we make use of real data and idealized nonhydrostatic numerical model simulations as well as linear theory to test the hypothesis that terrain variations in the height of the Sierra crest are responsible for the variability in the wave amplitude and characteristics in the along-barrier direction. Preliminary results suggest that wave launching is sensitive to relatively small-scale terrain variations along the Sierra crest and impact the wave amplitude and characteristics in the lower stratosphere. A series of high horizontal and vertical resolution COAMPS simulations will be used to explore the sensitivity to the along-barrier terrain variations. The three dimensional characteristics of gravity waves in the lower stratosphere and the sensitivity to the Sierra terrain will be examined from a linear theory perspective using multi-layer quasi-analytic linear solutions. Computations of the local group velocity will be used to identify the specific topographic features that correlate with the wave amplitude variations.